A customized computer is essentially one that has been built by hand from a mixed variety of separately built parts. What separates this from a prefabricated retail one is that it has been put together and checked thoroughly by the technician building it rather than slapped together by machines and an assembly line. It's similar to the difference between a factory printed motorcycle and a hand-built, sleek and shiny Harley-Davidson.
Custom built PCs have extensive hardware options to pick from, but to a non-enthusiast user that doesn't necessarily mean much. Perhaps you don't necessarily know the difference between all the different lines of hardware out there. That doesn't mean there aren't still several benefits of going with a custom PC that even non-savvy users can appreciate though.
Price - Because the parts used to build a custom PC can be purchased on discount or wholesale, the builder can usually sell the PC for a lower average price that still covers the cost of those parts compared to a prefab. They also have access to more efficient power supplies that will help you save on your power bill. Most retail PCs that offer similarly cheap prices usually falter in some pretty important areas like having dated or low quality hardware.
Performance and reliability - Retail PCs don't always use the best parts available on the market nor do they necessarily use correctly configured drivers and cabling. Custom PCs can outpace prefabs in longevity and performance for multiple reasons. They have the option for larger cases, more fans and better cable management. These three factors help cut down on the biggest factor in PC performance: heat. A hot computer is an unstable one, and retail machines are not necessarily designed to have optimal airflow. In short, retail PCs aren't built to last longer than a couple years.
Custom PCs also have access to the latest and greatest in heat distribution hardware such as water cooling and the top rated fan/heatsink combos.In addition to superior heat management, custom PCs can also provide you with the biggest bang for your buck. Want a PC that won't be outdated in only a few years without having spending a fortune? You don't have to with a custom PC!
Appearance - Custom built PCs offer the option to include things like case windows, custom colors and paint jobs, led lighting etc, all for the price they are actually worth. If you're a gaming enthusiast then you can appreciate having an awesome looking machine. Most retail PCs have a small, black case that looks bland and simply gets the job done. There are a few retail PCs out there with that extra 'cool factor' looks wise but they will cost you a pretty penny.
Freedom - Most retail PCs either come with a 'recovery' disc (which is not a replacement for a Windows install CD) or no CD at all. They're usually retrofitted with an OEM copy of Windows that cannot be installed on any other machine. These copies of Windows can sometimes contain alterations that limit what you can do with your machine.
Custom PCs almost always come with a retail copy of Windows, the disc it was installed with and the product key. All of these are essential and allow you to reformat and reinstall your OS if you need to. Without these you can end up having to purchase a new copy of Windows which can cost over a hundred pounds. You have the freedom to do what you want and alter what you want on your new PC. Of course, the biggest downfall of going with a custom PC is that they can be tricky to put together for someone who doesn't know what they're doing. That's what professionals are for though!